Effect of Non-fullerene Acceptors' Side Chains on the Morphology and

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Effect of Non-Fullerene Acceptors’ Side Chains on the Morphology and Photovoltaic Performance of Organic Solar Cells Cai'e Zhang, Shiyu Feng, Yahui Liu, Ran Hou, Zhe Zhang, Xinjun Xu, Youzhi Wu, and Zhishan Bo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09915 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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ACS Applied Materials & Interfaces

Effect of Non-Fullerene Acceptors’ Side Chains on the Morphology and Photovoltaic Performance of Organic Solar Cells Cai’e Zhang,‡a Shiyu Feng,‡b Yahui Liu,b Ran Hou,b Zhe Zhang,b Xinjun Xu,*b Youzhi Wu,*a Zhishan Bo*b a

School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou

730050, China. b

Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing

Normal University, Beijing 100875, China. KEYWORDS: non-fullerene acceptors, side chains, nanofibrils, organic solar cells, fused ring

ABSTRACT

Three indacenodithieno[3,2-b]thiophene (IT) cored small molecular acceptors (ITIC-SC6, ITIC-SC8 and ITIC-SC2C6) were synthesized and the influence of side chains on their performances in solar cells was systematically probed. Our investigations have demonstrated the variation of side chains greatly affects the charge dissociation, charge mobility and morphology

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of the donor:acceptor blend films. ITIC-SC2C6 with four branched side chains showed improved solubility, which can ensure the polymer donor to form favourable fibrous nanostructure during the drying of the blend film. Consequently, devices based on PBDBST:ITIC-SC2C6 demonstrated higher charge mobility, more effective exciton dissociation and the optimal power conversion efficiency up to 9.16% with an FF of 0.63, a Jsc of 15.81 mA cm-2, and a Voc of 0.92 V. These results reveal that the side-chain engineering is a valid way of tuning the morphology of blend films and further improving PCE in polymer solar cells.

Introduction Over the past decades, tremendous efforts have been devoted to stimulating the development of polymer solar cells (PSCs) considering their evident superiorities of light weight, solution processing, low cost and suitability for large-scale fabrication.1-8 Most PSCs employ a blend of an electron-withdrawing component and an electron-donating one as the active elements to generate photocurrent. Although fullerene derivatives like PC71BM and PC61BM have been extensively employed as the electron acceptor, their shortcomings including low absorption coefficient, morphological instability, and mismatching of energy levels between acceptor and donor limit further improvement in power conversion efficiency (PCE). In recent years, nonfullerene acceptors (NFAs), including fused-ring electron acceptors (FREAs),9 have been paid much attention, because of their excellent optical absorption properties and flexible energy level variability.10-14 High photovoltaic performance NFAs usually consist of three key components. The central aromatic fused rings can enhance the intermolecular charge transport; the electron-withdrawing


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moieties can adjust lowest unoccupied molecular orbital (LUMO) energy levels, provide a good stacking, and thus improve electron mobilities; and rigid side chains sticking out of molecular plane can supply the solubility and restrict the strong self-aggregation of the planar fused rings to prevent forming large crystalline domains. Among the reported NFAs, many efforts were devoted to changing the structure of conjugated backbones9,10,15-19 and electron-withdrawing end groups.20-25 As we know, in fullerene based PSCs, manipulating the side chains of polymers is of critical importance for enhancing the photovoltaic performance, since the type, position, size, and length of side chains can significantly influence the solubility, intermolecular interaction, charge-transport property and thin-film morphology.26-32 Therefore, studies of the side chain’s influence on the properties of NFAs are necessary and important, which may provide deeper insight into structure-property relationships. Currently, only little attention was paid to side chains attached to the central fused rings;33-36also, a clear picture that how side chains correlate with the photovoltaic performance of NFAs is absent. In addition, given that 4-(alkylthio)phenyl side chain has been widely employed in conjugated polymers,37-41 but its effect on the photovoltaic performance of NFAs is still unknown. In this work, a series of new non-fullerene acceptors (ITIC-SC6, ITIC-SC8 and ITICSC2C6) with a fused 7-heterocyclic ring (indacenodithieno[3,2-b]thiophene, abbreviated as IT) core, two 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) terminals, and four 4(alkylthio)phenyl side chains were designed and synthesized (Scheme 1), aiming to investigate side chain’s effect on the photovoltaic performance. Our results have demonstrated that the 4(alkylthio)phenyl side chains of the NFAs can tune the energy levels, exciton dissociation ability, film morphology and photovoltaic performance. Specially, morphology characterizations revealed that when the acceptor has a better solubility in the processing solvent, the polymer


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donor is more prone to aggregate to form nanofibrils. Consequently, PSCs fabricated with the blends of the polymer donor PBDB-ST and ITIC-SC2C6 with four branched substituents exhibited well-distributed nanofibrillar interpenetrating networks, achieving a PCE up to 9.16% with an FF of 0.63, a Jsc of 15.81 mA cm-2, and a Voc of 0.92 V, which is higher than that of acceptors with linear side chains. These results indicate that side-chain engineering is an effective way to further regulate the photoelectric properties of acceptors and the morphology of blends to obtain a higher PCE.

Scheme 1. Synthetic route to ITIC-SC6, ITIC-SC8 and ITIC-SC2C6. Reagents and conditions: (i) n-BuLi, tetrahydrofuran, −78 °C; (ii) BF3·Et2O, DCM, room temperature; (iii) n-BuLi, −78 °C, tetrahydrofuran, DMF; (iv) pyridine, chloroform, reflux. Results and Discussion Material Synthesis and Characterization. The synthetic procedure of ITIC-SC6, ITIC-SC8 and ITIC-SC2C6 are shown as Scheme 1. Compound 1, which was prepared according to the reported procedures, was used as the starting material.42 The treatment of compound 1 with an


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excess of 4-(alkylthio)phenyllithium, which was previously prepared by the reaction of 4-bromo1-(alkylthio)benzene and n-butyllithium, afforded the corresponding diol. Without further purification, the diol was directly used for the ring-closure reaction by treatment with BF3 etherate to generate the ladder-type intermediate 2. The abstraction of α-hydrogen of compound 2 by reaction with n-butyllithium afforded the dianions, which were subsequently quenched by dimethylformamide (DMF) to afford compound 3. Followed by one-step simple Knoevenagel condensation with INCN, three new ITIC derivatives (ITIC-SC6, ITIC-SC8 and ITIC-SC2C6) were obtained in yields of 58% to 60%. Each new compound was thoroughly characterized by elemental analysis and 1H and 13C NMR spectroscopy (Experimental Section). These three ITIC derivatives can be readily dissolved in dichloromethane (DCM), o-dichlorobenzene (DCB), and chloroform et al. at room temperature. Thermal properties of these three acceptors were attained by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) (heating rate: 10 °C/min, under a nitrogen atmosphere). As shown in Figure S1, TGA traces indicated that ITIC-SC6, ITIC-SC8 and ITIC-SC2C6 have a good thermal stability with the 5% decomposition temperature of 326, 341 and 324 °C, respectively. No obvious glass transition was detected in DSC curves in the range of 25 to 350 °C. The packing of ITIC-SC6, ITIC-SC8 and ITIC-SC2C6 in films was surveyed by X-ray diffractions (XRD) measurement. As displayed in Figure 1, the diffraction peaks of ITIC-SC6, ITIC-SC8 and ITIC-SC2C6 are located at 2θ of 5.52°, 5.04° and 5.00°, corresponding to lamellar distances of 16.01, 17.53 and 17.67 Å, respectively. Diffractions arising from the π-π stacking of backbones appeared at 2θ of 26.20°, 26.05° and 25.99°, corresponding to distances of 3.40, 3.42, and 3.43 Å for ITIC-SC6, ITIC-SC8 and ITIC-SC2C6, respectively. The consequence indicated that the acceptor with


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branched side chains possesses slightly increased lamellar and π-π stacking distances relative to those with the linear ones.

Figure 1. XRD curves of PBDB-ST, acceptors and their corresponding blend films, (a) ITICSC6, (b) ITIC-SC8 and (c) ITIC-SC2C6. Photophysical and Electrochemical Properties. UV-vis absorption spectra of ITIC-SC6, ITIC-SC8, ITIC-SC2C6 and the polymer donor PBDB-ST recorded in DCB solutions and in films are displayed in Figure 2a and 2b, respectively. All these new acceptors exhibit a strong absorption band ranged in 500 to 750 nm with a well-defined absorption peak at approximately 665 nm in dilute DCB solutions, which is ascribed to the intramolecular charge transfer band.43 In films, significant bathochromicshifts of 48, 54 and 40 nm were observed for ITIC-SC6, ITIC-SC8 and ITIC-SC2C6, respectively, which can be attributed to the enhanced Jaggregation when going from solutions to films. Besides, from Figure 2b we can see, the acceptors demonstrate complementary absorption with the polymer donor PBDB-ST, which is in favour of extending the light absorption.


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Figure 2. Normalized ultraviolet-visible absorption spectra of PBDB-ST, ITIC-SC6, ITIC-SC8 and ITIC-SC2C6 (a) in DCB solutions, (b) in film states; (c) energy levels of the donor and acceptors. As shown in Figure S2, cyclic voltammetry (CV) was employed to determine the electrochemical properties of these three desired acceptors. The onset oxidation/reduction potentials (Eox/Ered) of ITIC-SC6, ITIC-SC8 and ITIC-SC2C6 are +0.97/−0.80, +0.99/−0.81 and +1.03/−0.85 V, respectively. Following the equation EHOMO/LUMO = −e[Eox/red, onset − E(Fc/Fc+) + 4.8],44 the HOMO/LUMO energy levels (EHOMO and ELUMO) were estimated to be −5.68/−3.91 eV for ITIC-SC6, −5.70/−3.90 eV for ITIC-SC8, and −5.74/−3.86 eV for ITIC-SC2C6, demonstrating that the acceptor with branched side chains has a slightly shallower LUMO level and a wider bandgap than those with linear ones (Figure 2c).


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Figure 3. (a) Device architecture and chemical structures of the components used in the active layer. (b) J-V curvesand (c) EQE curves of the optimized PBDB-T:ITIC-SC6/SC8/SC2C6 based devices. Photovoltaic Properties. Photovoltaic properties of ITIC-SC6, ITIC-SC8 and ITIC-SC2C6 were investigated in devices with a configuration of ITO/zinc oxide (30 nm)/active layer (80 nm)/MoO3 (8.5 nm)/Ag (100 nm) (Figure 3a). To achieve the best photovoltaic performance, a series of conditions were systematically screened, including the concentration of blended films, the weight ratio of donor to acceptor, the volume of o-chlorobenzaldehyde (CBA) additive and the spin-coating speed. The current density-voltage (J-V) characteristics of PSCs are displayed in Figure 3b and photovoltaic parameters are summarized in Table 1. The optimal ratio of donor (PBDB-ST) to acceptor (ITIC-SC6, ITIC-SC8 or ITIC-SC2C6) is 1:1 (weight by weight, w/w), and the optimized thickness of the active layer is about 80 nm. The active layer is deposited at a spin coating rate of 1400 r/min from their dilute DCB solutions (concentration: 8 mg mL-1). As displayed in Figure 3b, photovoltaic devices fabricated with the blends of PBDBST and ITIC-SC6 provided a PCE of 6.54% with an FF of 0.55,a Jsc of 13.52 mA cm-2, and a Voc of 0.88 V. After the addition of CBA (0.1% by volume) as the processing additive, the PCE was slightly enhanced to 7.27% with an FF of 0.58, a Jsc of 13.92 mA cm-2, and a Voc of 0.90 V, which could be assigned to the optimized morphology of the active layer after the addition of CBA to the blend (vide infra). For ITIC-SC8 based devices, a PCE of 7.09% with an FF of 0.57, a Jsc of 13.97 mA cm-2, and a Voc of 0.89 V was achieved. In addition, a slightly enhanced PCE of 7.79% was also gained with Voc, Jsc and FF of 0.90 V, 14.43 mA cm-2 and 0.60, respectively, after the introduction of CBA (0.2% by volume). For ITIC-SC2C6 based devices, they exhibited a PCE of 7.96% with an FF of 0.60, a Jsc of 14.42 mA cm-2, and a Voc of 0.92 V, and when using


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pure DCB as the solvent. Notably, when 0.3% CBA was employed as the additive for DCB, the PCE was greatly improved to 9.16% with an FF of 0.63, a Jsc of 15.81 mA cm-2, and a Voc of 0.92 V. We infer that the slight enhancement in Voc values probably benefits from the improved charge transfer and superior morphology of the active layer after the introduction of CBA, which decrease energy loss from charge recombination. The increased Jsc and FF of ITIC-SC2C6 based devices relative to ITIC-SC6 or ITIC-SC8 based ones arise from the higher charge mobility and better morphology of the donor:acceptor blends (vide infra). Figure 3c shows the external quantum efficiencies (EQEs) of the ITIC-SC6, ITIC-SC8 and ITIC-SC2C6 based optimized PSCs. The Jsc values obtained by integrating the EQE curves with an AM1.5 G reference spectrum are in accord with those obtained from J-V measurements (within 4% mismatch). All devices displayed a broad photo-to-current response from 300 to 800 nm, indicating that polymer donor and non-fullerene acceptors contribute together to the Jsc values. The highest EQE values of ITIC-SC6, ITIC-SC8 and ITIC-SC2C6 reached 65%, 68% and 70%, respectively, indicating effective photon harvesting and carrier collection in the active layers. Table 1 Photovoltaic parameters of PBDB-ST:ITIC-SC6, PBDB-ST:ITIC-SC8 and PBDBST:ITIC-SC2C6 based optimized devices under AM1.5G illumination , 100 mW cm-2. Acceptor

CBA Voc Jsc FF (%) (V) (mA cm-2) --0.88 ± 0.00 13.66 ± 0.40 0.56 ± 0.01 ITIC-SC6 0.1 0.90 ± 0.00 13.87 ± 0.12 0.58 ± 0.01 --0.89 ± 0.01 13.73 ± 0.46 0.58 ± 0.01 ITIC-SC8 0.2 0.90 ± 0.00 14.43 ± 0.19 0.60 ± 0.01 --0.92 ± 0.00 14.22 ± 0.23 0.61 ± 0.01 ITIC-SC2C6 0.3 0.92 ± 0.00 15.32 ± 0.50 0.64 ± 0.01 a The statistical parameters were obtained from six devices.

PCE (%) best/avea 6.54 7.27 7.09 7.79 7.96 9.16

6.47 ± 0.05 7.17 ± 0.09 6.90 ± 0.12 7.49 ± 0.19 7.69 ± 0.25 8.94 ± 0.21


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Charge Transport Characteristics. The charge transport properties of the active layer were evaluated by space charge limited current (SCLC) method using electron- and hole-only devices with configurations of FTO (cathode)/active layer (80 nm)/Al (anode) and

ITO

(anode)/PEDOT:PSS (30 nm)/active layer (80 nm)/Au (cathode), respectively. As shown in Figure S3, the hole (µh)/electron (µe) mobilities were calculated to be 1.25×10-4 /1.34×10-4 cm2V1 -1

s for ITIC-SC6 based blends, 1.64×10-4 /1.76×10-4 cm2V-1s-1 for ITIC-SC8 based blends and

2.91×10-4 /2.69×10-4 cm2V-1s-1 for ITIC-SC2C6 based blends. After the addition of CBA, the µh/µe mobilities of ITIC-SC6, ITIC-SC8 and ITIC-SC2C6 based blends were increased to 2.43×10-4 /2.15×10-4, 3.13×10-4 /2.93×10-4 and 5.62×10-4 /5.43×10-4 cm2V-1s-1, respectively. A higher mobility could facilitate charge transport and reduce the possibility of charge recombination, leading to an increased Jsc and thus a higher PCE of the corresponding device.45 In order to further elucidate the charge dissociation and charge transfer behavior in the active layer, photoluminescence (PL) quenching experiment was then carried out. As shown in Figure 4, the PL spectra were effectively quenched by 87% in the PBDB-ST:ITIC-SC6 blend film and 94% in the PBDB-ST:ITIC-SC8 blend film. Significantly, the PBDB-ST:ITIC-SC2C6 blend film exhibited over 98% fluorescence quenching, suggesting more effective photo-induced charge transfer and exciton dissociation in the active layer.

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Figure 4. Photoluminescence spectra of neat acceptor (excitation at 670 nm) and PBDBST:acceptor (excitation at 670 nm) in film: (a) ITIC-SC6, (b) ITIC-SC8 and (c) ITIC-SC2C6. Morphology Characterization. Atomic force microscopy (AFM) with tapping-mode was utilized to obtain the surface morphology of blend films. As illustrated in Figure S4, all blend films without the CBA addition showed a smooth surface. The corresponding root-mean-square (RMS) roughness values of ITIC-SC6, ITIC-SC8 and ITIC-SC2C6 based films (without additives) are 0.92, 0.92 and 1.00 nm, respectively, and slightly increase to 1.26, 1.17 and 1.11 nm, respectively, after the CBA addition. To understand the morphology differences and their influence on photovoltaic performance, as shown in Figure 5, transmission electron microscopy (TEM) were used to further characterize the blend films. PBDB-ST:ITIC-SC6 blend film (without CBA addition) exhibited very uniform morphology. In contrast to ITIC-SC6, some fibrillar structures with nanoscale phase separation were formed in the PBDB-ST:ITIC-SC8 blend film (without CBA addition) while more significant nanofibers could be clearly observed in the PBDB-ST:ITIC-SC2C6 blend film, suggesting that the side chains of non-fullerene acceptors exert an evident influence on the nanoscale morphology of active layers. These could be explained by the effect of the size of side chains on the solubility of acceptors. As we know, the solubility of acceptors increases in the order of ITIC-SC6, ITIC-SC8, and ITIC-SC2C6. During the drying of the blend films, good solubility of acceptors in the processing solvent can keep them still in solution before the polymers precipitating to form favorable fibrous structure.46,47 After the addition of CBA, for all blend films, more homogeneous morphology with nanoscale phase separation can be observed. Such a morphology is beneficial to charge transportation, thus in favor of higher device performance. The above results demonstrated that


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high-performance organic photovoltaic devices could be obtained through side-chain engineering of acceptor materials.

Figure 5. TEM images of PBDB-ST:ITIC-SC6 blends (a) in pure DCB (d) in DCB with 0.1% CBA additive; PBDB-ST:ITIC-SC8 blends (b) in pure DCB, (e) in DCB with 0.2% CBA additive and PBDB-ST:ITIC-SC2C6 blends (c) in pure DCB, (f) in DCB with 0.3% CBA additive. The scale bar is 200 nm for (a), (b) and (c) and 100 nm for (d), (e) and (f). Conclusions To sum up, a series of fused-ring ladder-type small molecules ITIC-SC6, ITIC-SC8 and ITIC-SC2C6 with different 4-(alkylthio)phenyl side chains have been synthesized and exploited as acceptor materials for non-fullerene PSCs. The results demonstrate that variation of side chains for these acceptors can influence electronic energy levels, charge dissociation, charge mobility and film morphology, ultimately resulting in different photovoltaic performance. TEM investigations revealed that polymer is more prone to form fibrous structure when blended with an acceptor that carrying branched side chains. Consequently, ITIC-SC2C6 based devices exhibited the optimal PCE of 9.16% with an FF of 0.63, a Jsc of 15.81 mA cm-2, and a Voc of 0.92 V. Our results provide new insights into the structure design of non-fullerene acceptor


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materials to achieve well-performing morphology and high PCE of devices via side chain engineering.

ASSOCIATED CONTENT Supporting Information. This supporting information is available free of charge on the ACS Publications website. Synthesis and characterization of acceptors, TGA, CV, SCLC, AFM measurement and OPV fabrication. AUTHOR INFORMATION Corresponding Author * Email: [email protected], [email protected], [email protected] Author Contributions ‡ Cai’e Zhang and Shiyu Feng made same contributions to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the NSF of China (21574013, 51673028) and the Fundamental Research Funds for the Central Universities are gratefully acknowledged. REFERENCES


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TOC


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Effect of Non-fullerene Acceptors' Side Chains on the Morphology and

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